I have always heard that a horse should not be jumped under saddle until it is at least 4. I would wait longer, probably more towards 5, but 4 is what I have heard. As far as the age to stop jumping, it really depends on the horse and what you are doing.
For instance, a big eventer or show jumper will probably retire before a horse who jumps every now and then in 3' hunters. If your horse has arthritis or any major joint problem, retirement will be earlier than one without. I know horses who have been jumped well into their 20's. It really depends on the horse. I can't say a certain age.
It varies from horse to horse. Certain breeds mature faster than others, as do individuals. Everyone has a different opinion on the matter too that range from starting at 3 to starting at 6 or 7. It hasn't been proven that jumping early is going to ruin your horse in the long run, but excessive jumping, (or excessive anything for that matter) will cause wear and tear. I know some people who jump twice a week, but I also know other people who compete in the 3'6-4' range who school over fences twice a month tops during competition season.
Horses aren't fully grown until about 4 years old, and I believe you shouldn't even SIT on a horse's back until it's physically mature. So, if you start flatwork around age 4, jumping shouldn't happen until at LEAST age 5. :)
Many people start horses younger, and that's fine, unless you don't want your horse lame by the time it's 12. I personally want my horses to last well into their 20's. ;)
I like to start training the basics at 4 like walk to a halt. Halt to a walk. Walk to trot. Trot to walk. Trot to canter. Canter to trot. Etc. Then when they turn 5, then start working them with jumps. I have a 23 year old OTTB. Whom I retired not because he can't jump but because he has been jumping for 16 years and raced for the first 3. So I always say in their early 20s. Though it is my opinion.
My coaches share a 21/22 year old Australian TB who is still competing lightly in the 1.1/1.15m division.
I don't mind starting a horse early, as long as it is light easy stuff, with maybe a turn out of a few months once they have the basics down pat. Basics to me are a walk-trot-canter including light transitions, a good halt, and to stand when mounting and dismounting.
I agree that a horse will tell you when they are done. If all of a sudden they are refusing fences they normally wouldnt blink an eye at, it might be time to have them looked at to figure out why they are doing it, and possibly back down to lighter, easier work.